Applicants hereby claims purusant to 35 U.S.C. xc2xa7 119 the priority of French Patent Application No. 97 10325 filed in the French Patent Office on Aug. 13, 1997.
The present invention relates to a geotextile structure for filtration.
More precisely, the invention relates to a geotextile structure performing the standard functions for such structures, i.e. the stabilization of soils or of other equivalent materials and which presents, moreover, such filtration properties as to allow passage only of liquids impregnating the soil and not the soil itself or other types of waste.
Geotextile stucture used for filtering soils are well known. They are most often constituted by woven or non-woven layers of filaments or of fibers of synthetic materials such as polypropylene, the presence of this structure in contact with the soil, or buried in the soil, ensuring stability thereof.
The present invention concerns a structure of this type which, moreover, presents characteristics such that they allow a filtration of the liquids present in the ground.
The technique of filtration with the aid of geotextiles has existed for a long time. The principle consists in retaining the soil to be filtered while allowing the water to pass without pressure drop or with a pressure drop which is a low as possible. Taking into account the granulometry of natural soils, it is not possible to retain all the particles of the soil. This is why the object of the present dimensioning of such filters is only to retain the skeleton of the soil. These are the largest particles of the soil which may retain the finest particles by creation of an xe2x80x9cauto-filterxe2x80x9d within the soil. This mechanism therefore supposes that, in the geotextile, the finest particles pass therethrough without clogging it. The same applies for the particles of soil in suspension which may come into contact with the geotextile when employed, for example in the presence of mud.
It is generally admitted that the best performances are obtained by needled geotextiles, due to their three-dimensional behaviour. However, as has been demonstrated, the geotextile product must, in order to perform its functions of filtration, present a minimum number of constrictions (i.e. of spaces created by three filaments or more through which the particle of soil must pass) in its thickness, in order to have a high statistical homogeneity of local filtration openings.
However, it has proved that, in order to withstand the aggressions when such a filtration geotextile is used, it is necessary to increase the mass of the product used to obtain the necessary thickness after the damage resulting from employment. This increase in mass, combined with the minimum number of constrictions necessary, has for an effect to considerably increase the risk of internal clogging of the product. It will be appreciated that such a clogging considerably reduces the filtering properties of the geotextile texture.
It is an object of the present invention to provide a geotextile structure which effectively perfoms the desired function of filtration while allowing a sufficient mechanical protection of the structure to ensure integrity thereof when put to use.
To attain this object, according to the invention, the geotextile structure for filtration is characterized in that it comprises:
a lower layer of geotextile material for filtration presenting a number of constrictions n included between 15 and 40 and made of synthetic filaments or fibers presenting a count D (decitex); and
an upper layer of geotextile material for protection made of synthetic filaments or fibers presenting a count at least equal to 2D (decitex), said upper and lower layers being bonded to each other.
For example, this count is equal to 4D (dexitex).
It will be appreciated that, thanks to the combination of the lower layer for filtration, which is optimalized for the purposes of this function, the overall structure effectively performs the function of filtration under the desired conditions. It will also be appreciated that, thanks to the upper layer of synthetic material which presents a mass adapted to the desired resistance, the lower filtration layer is mechanically protected while avoiding the risks of clogging of the filtration layer.
According to a preferred characteristic, the number of constrictions of the lower layer is included between 20 and 35.
Likewise preferably, the count of the filaments of the upper layer is included between 3D (decitex) and 4D (decitex).